Semiconductor Barrier Research Articles

Hg/Molecular Monolayer−Si Junctions: Electrical Interplay between Monolayer Properties and Semiconductor Doping Density

Metal−organic molecule−semiconductor junctions are controlled not only by the molecular properties, as in metal−organic molecule−metal junctions, but also by effects of the molecular dipole, the dipolar molecule−semiconductor link, and molecule−semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current−voltage, capacitance−voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled ju...

Tunnelling transport in nonuniform barriers as a possible way for current control

This paper presents the results of computer simulation of the under barrier tunnelling transport through the nonuniform heterostructure barriers containing spherical or cylindrical nano objects. Embedding of nano objects in the barrier enables one to construct the semiconductor barriers with predefined properties. Application of external potential to these objects exponentially changes the current through the barrier and it can be used for current control and amplification of signals.

Effect of Rashba spin–orbit coupling on the spin polarized transport in diluted magnetic semiconductor barrier structures

We investigate theoretically the effects of Rashba spin–orbit coupling on the spin dependent transport through diluted magnetic semiconductor single and double barrier structures in the presence of a magnetic field. We find that the Rashba spin–orbit coupling gives rise to an enhancement of the negative tunnelling magnetoresistance of the diluted magnetic semiconductor single barrier structure and a pronounced beating pattern in the tunnelling magnetoresistance and spin polarization of the diluted magnetic semiconductor double barrier structure.

Spin trajectory along an evanescent loop in zinc-blende semiconductors

Due to the absence of inversion symmetry, spin-orbit interaction leads to a very particular topology of the evanescent states in zinc-blende semiconductors, which may consist of loops connecting different spin sub-bands at the zone center. The spin-vector motion along such loops is analytically or numerically studied. A surprising picture emerges from this detailed analysis. Namely, the two spin sub-bands do not correspond to opposite spin states near the Brillouin-zone center and merge with identical spins at larger wave vector. This determines the spin-filtering capabilities of the semiconductor barrier.

Magnetism of semiconductor-based magnetic tunnel junctions under electric field from first principles

Semiconductor magnetic tunnel junctions (MTJs), composed of diluted magnetic semiconductors (DMSs) sandwiching a semiconductor barrier, have potential applications in spintronics but their development has been slow due to the difficulty of controlling the magnetism of DMSs. In terms of density functional calculations for model semiconductor MTJs, (Zn,Co)O/ZnO/(Zn,Co)O and (Ga,Mn)N/GaN/(Ga,Mn)N, we show that the magnetic coupling between the transition metal ions in each DMS electrode of such semiconductor MTJs can be switched from ferromagnetic to antiferromagnetic, or vice versa, under the application of external electric field across the junctions. Our results suggest a possible avenue for the application of semiconductor MTJs.

Spin rotation, spin filtering, and spin transfer in directional tunneling through barriers in noncentrosymmetric semiconductors

We discuss possible tunneling phenomena associated with complex wave vectors along directions where the spin degeneracy is lifted in noncentrosymetric semiconductors. We show that the result drastically depends on the direction. In the [110] direction, no solution can be calculated in the usual way assuming that the wave function and its derivative are continuous. A method for obtaining physical solutions is given and consequences are drawn. As a result, there is no spin filtering in such a direction but the spin undergoes a precession through the barrier with the rotation angle being proportional to the barrier thickness. In a direction close to [001] we find a spin-filter effect in close agreement with the model discussed by Perel' et al. [Phys. Rev. B 67, 201304(R) (2003)].

Unified tunnelling-diffusion theory for Schottky and very thin MOS structures

We derive general formulae for calculating the transport of free charge carriers in a MOS structure with a thin insulating layer. In particular, we obtain relationships for boundary concentrations of free charge carriers on the insulator–semiconductor interface and for the current densities flowing through the MOS structure. Our direct tunnelling-diffusion approach makes the well known thermionic emission–diffusion theory for the Schottky interface applicable also to metal–insulator–semiconductor barriers with a very thin insulator layer. We demonstrate how direct tunnelling through the insulating layer and drift–diffusion of free charge carriers in the semiconductor affect the I–V and C–V curves and the boundary concentrations needed to numerically solve the continuity equations.

Effects of the barrier metal thickness and hydrogen pre-annealing on the characteristic parameters of Au/ n-GaAs metal–semiconductor Schottky contacts

It is important to investigate the factors that influence the metal–semiconductor interfaces. Some of these factors are the effects of the semiconductor surface and barrier metal. Therefore, in this work we have investigated the influences of hydrogen pre-annealing and barrier metal thickness on the Au/ n-GaAs Schottky barrier diodes. Having performed current–voltage and capacitance–voltage measurements, the values of the ideality factor and barrier height for the un-annealed diodes range from 1.14 and 0.855 eV (for 5 nm) to 1.08 and 0.794 eV (for 100 nm), respectively. Also, the same parameters for the H 2 pre-annealed diodes range from 1.78 and 0.920 eV (for 5 nm) to 1.11 and 0.774 eV (for 100 nm), respectively.

Rashba spin–orbit effect on dwell time of electrons tunneling through semiconductor barrier

Abstract A study on characteristics of electrons tunneling through semiconductor barrier is evaluated, in which we take into account the effects of Rashba spin–orbit interaction. Our numerical results show that Rashba spin–orbit effect originating from the inversion asymmetry can give rise to the spin polarization. The spin polarization does not increase linearly but shows obvious resonant features as the strength of Rashba spin–orbit coupling increases, and the amplitudes of spin polarization can reach the highest around the first resonant energy level. Furthermore, it is found that electrons with different spin orientations will spend quite different time through the same heterostructures. The difference of the dwell time between spin-up and spin-down electrons arise from the Rashba spin–orbit coupling. And it is also found that the dwell time will reach its maximum at the first resonant energy level. It can be concluded that, in the time domain, the tunneling processes of the spin-up and spin-down electrons can be separated by modulating the strength of Rashba spin–orbit coupling. Study results indicate that Rashba spin–orbit effect can cause a nature spin filter mechanism in the time domain.

Reply to “Comment on ‘Spin-dependent tunneling through a symmetric semiconductor barrier: The Dresselhaus effect’ ”

The present paper is the reply to Sandu's Comment on our paper [Phys. Rev. B 72, 153314 (2005)], i.e., the effect of the current operator on the spin-dependent tunneling through a barrier in the presence of the Dresselhaus spin-orbit interaction (DSOI). We demonstrate theoretically and numerically that our previous numerical result is correct when there is no DSOI in the contact region and it remains a good approximation in the presence of the ${k}^{3}$-DSOI in the contact regions.

SOFTWARE FOR INVETIGATION OF CURRENT TRANSPORT PROCESSES IN SEMICONDUCTOR STRUCTURES

The paper reports the development of the software “Solar Cell Simulator”, designed for numerical modeling of current transport phenomena in the different semiconductor barrier structures depending on the type and magnitude of external and internal parameters (such as intensity and spectrum of illumination, applied voltage, type and parameters of semiconductor materials, impurity concentration, etc.).

Effect ofsp−dexchange interaction on excitonic states inCdSe∕ZnSe∕Zn1−xMnxSequantum dots

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral $\mathrm{CdSe}∕\mathrm{ZnSe}∕\mathrm{ZnMnSe}$ quantum dots (QDs) with various magnitudes of $sp\text{\ensuremath{-}}d$ exchange interaction has been investigated in magnetic fields $B$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnitude of the $sp\text{\ensuremath{-}}d$ interaction is varied by changing the penetration of the exciton wave function ${\ensuremath{\psi}}_{X}$ into the diluted magnetic semiconductor (DMS) barrier, $\ensuremath{\eta}$, by variation of the nonmagnetic $\mathrm{ZnSe}$ barrier layer thickness about a value of $1.75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The small penetration of ${\ensuremath{\psi}}_{X}$ into the DMS barrier allowed a decrease of the X and XX emission line broadening caused by magnetic fluctuations and resolution of the fine structure of X states in a single QD. In contrast to bulk DMSs, the exciton line broadening when $B$ is normal to the QD plane was found to be a nonmonotonic function of magnetic field: at high $B$ it is suppressed due to Mn ion spin alignment, whereas at low $B$ it is decreased due to the mixing of ${J}_{z}=+1$ and $\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{X}$ states in low-symmetry QDs by the electron-hole $(e\text{\ensuremath{-}}h)$ exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the $e\text{\ensuremath{-}}h$ exchange interaction at $B=0$ and (ii) strong enhancement of spin relaxation between ``bright'' $J=1$ exciton states in magnetic fields normal to the QD plane already at $\ensuremath{\eta}\ensuremath{\sim}1%$. The spin relaxation from bright to ``dark'' exciton states is negligible up to $\ensuremath{\eta}\ensuremath{\sim}4%$. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at $B=0$ already at $\ensuremath{\eta}\ensuremath{\sim}4%$.

Development of Two Spectrometers for the Noiseless β Energy Spectrum of Radiocarbon at Room Temperature

In this work, two spectrometric methods are described to have a noiseless β energy spectrum of radiocarbon (14C) as low‐energy β emitter. The spectrometers use two timing techniques with a silicon (Si) surface barrier semiconductor detector. The experimental results are compared; they show that the methods followed in the present work were successful for obtaining a noiseless β energy spectrum.

Negative differential resistance of electrons in graphene barrier

The graphene is a native two-dimensional crystal material consisting of a single sheet of carbon atoms. In this unique one-atom-thick material, the electron transport is ballistic and is described by a quantum relativisticlike Dirac equation rather than by the Schrödinger equation. As a result, a graphene barrier behaves very differently compared to a common semiconductor barrier. The authors show that a single graphene barrier acts as a switch with a very high on-off ratio and displays a significant differential negative resistance, which promotes graphene as a key material in nanoelectronics.

Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.

Simulation of a tunable optically pumped terahertz intersubband laser with diluted magnetic semiconductors

A simulation of an optically pumped laser based on a ZnSe∕Zn1−yCdySe double quantum well with a Zn1−xMnxSe diluted magnetic semiconductor barrier is presented. Giant Zeeman splitting in diluted magnetic semiconductors leads to splitting of electronic states, which in turn leads to tunability of laser wavelength by external magnetic field. Tunability is predicted throughout the wavelength range between 60 and 72μm at low temperatures.

Proposal for an electrical spin cell with single barrier

We propose a spin cell based on photon-assisted tunneling through a conventional semiconductor barrier. The Dresselhaus spin-orbit interaction is included to break the spin rotation symmetry. Due to the in-plane electric field induced asymmetric momentum distribution in one lead, continuous flows of spin currents are driven through a barrier by an ac field. The net charge current remains zero. The spin current via photon-assisted tunneling can be readily adjusted via tuning the ac frequency or the in-plane electric field. This device may function as an ideal spin cell to supply spin currents in the spintronics circuit.

Angular dependence of tunneling magnetoresistance in magnetic semiconductor heterostructures

Theoretical studies on spin-dependent transport in magnetic tunnel heterostructures consisting of two diluted magnetic semiconductors (DMS) separated by a nonmagnetic semiconductor (NMS) barrier, are carried in the limit of coherent regime by including the effect of angular dependence of the magnetizations in DMS. Based on parabolic valence band effective mass approximation and spontaneous magnetization of DMS electrodes, we obtain an analytical expression of angular dependence of transmission for DMS/NMS/DMS junctions. We also examine the dependence of spin polarization and tunneling magnetoresistance (TMR) on barrier thickness, temperature, applied voltage and the relative angle between the magnetizations of two DMS layers in GaMnAs/GaAs/GaMnAs heterostructures. We discuss the theoretical interpretation of this variation. Our results show that TMR of more than 65% are obtained at zero temperature, when one GaAs monolayer is used as a tunnel barrier. It is also shown that the TMR decreases rapidly with increasing barrier width and applied voltage; however at high voltages and low thicknesses, the TMR first increases and then decreases. Our calculations explain the main features of the recent experimental observations and the application of the predicted results may prove useful in designing nano spin-valve devices.

Polarization of electron spin in two barrier system based on semimagnetic semiconductors

AbstractThe spin‐dependent tunneling of electrons through a two barrier semiconductor heterostructure with a semimagnetic layer was investigated. It was shown that the resonant level splitting in the semimagnetic well under an external magnetic field allows achieving a high level of spin polarization of the current flowing through the proposed spin filter. The dependence of the polarization depth on the parameters of the sample was calculated in the two component diffusion transport model. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast spin injection from Cd 1−xMn xTe magnetic barriers into a CdTe quantum well studied by pump–probe spectroscopy

Spin injection from diluted magnetic semiconductor (DMS) barriers of Cd 1− x Mn x Te into a quantum well (QW) of CdTe is studied, by means of pump–probe absorption spectroscopy in magnetic fields. Fast decay characteristics of circularly polarized differential absorbances of spin-polarized excitons in the DMS barrier show the exciton injection time of 6 ps from the barriers into the QW. In accordance with the fast relaxation of the spin-polarized excitons from the barrier, we observe the rise of circular polarization degree for the differential absorption of the CdTe QW in magnetic fields, evidently indicating the spin injection. In addition, the circular polarization degree up to 0.3 is developed in the well immediately after pumping, originating from the fast relaxation of a heavy hole (hh) spin less than 0.2 ps, due to the giant Zeeman effect caused by the penetration of the hh wave function into the DMS barriers.